The present invention relates to a window-passage detection system of an airplane for detecting a timing when the airplane passes through a window frame, that is, an appointed space including a category decision height where a final landing decision should be taken when the airplane is to be landing according to precision approach making use of a satellite navigation system, at airborne side and instantaneously at the timing.
Since 1993, the application of GPS (Global Positioning System) to the precision approach landing of the airplane has been studied at AWOP (All Weather Operating Panel) of the ICAO (International Civil Aviation Organization), and RNP (Required Navigation Performance) is proposed there for defining necessary performance for the precision approach landing.
Referring to FIG. 6, a precision approach landing system making use of DGPS (Differential Global Positioning System) is described, which is considered to give a highest measurement precision among actually available satellite navigation systems.
The DGPS is a technique to improve measurement precision by revising three-dimensional information obtained at a mobile station making use of the pseudo-distance error measured at and transmitted from a reference position whereof precise three-dimensional position is known. Therefore, the transmission of the pseudo-distance error defines the measurement precision of the system.
The DGPS system of FIG. 6 comprises a ground system (reference station) 101 and an airborne system 102. The ground system 101 calculates the pseudo-distance and its differential of each satellite from positional information received by a GPS receiver 104 through a GPS antenna 103 with a computer 105 based on the precise three-dimensional position of the ground system 101 which is beforehand measured. Correction data 106 obtained from the pseudo-distance and its differential is transmitted from a transmission antenna 108 through a data transmitter 107.
In the airborne system 102, positional information calculated from satellite signals received by a GPS receiver 113 through a GPS antenna 112 and correction data 110 received from the ground system (reference station) 101 by a data receiver 111 through a data reception antenna 109 are supplied to a computer 114. The computer 114 obtains its precise position by revising the positional information making use of the correction data 110.
According to experimental data presented in a paper entitled "An Experiment of Approach and Landing according to DGPS", pp. 13-16, proceedings of 28-th symposium of Electronic Navigation Research Institute, Ministry of Transport, the measurement error was about .+-.5 m in the DGPS system wherein a narrow correlator receiver is applied to each of the GPS receivers 104 and 113, and the correction data 106 is measured and transmitted every 5 seconds according to data format (ASCII) of the GPS receiver 104 from the data transmitter 107 to the data receiver 111 by way of spread spectrum modulation.
On the other hand, the allowable height deviation is .+-.4.5 m at 30 m height for RNP category 2, and it is .+-.1.5 m at 15 m height for RNP category 3A, according to the AWOP proposal beforehand described.
Therefore, the DGPS system above described can not give sufficient exactness of cm-order necessary for the precision approach landing of RNP category 2 or RNP category 3A, being unable to exactly detect passage of the inner window frame including the decision height of 30 m of the RNP category 2 or that of 15 m of the RNP category 3A, at airborne side.
It may be considered to improve precision of the DGPS system for dealing with this problem. However, the DGPS systems, which are provided at reference points whereof three-dimensional position is known for transmitting GPS correction data to the airborne users by always measuring pseudo-distance errors of the GPS signals, need considerable maintenance cost without saying of their installation cost. Therefore, further precision improvement of the DGPS systems would require enormous cost.